The role of ultrafine particles (UFPs) from a human health perspective is increasingly being recognized (See Ibald-Mulli et al., 2002; and Oberdorster and Utell, 2002, both hereby incorporated herein by reference.), and the interaction of these particles with their environment is dependent on, among other parameters, their size and number concentration (See Penttinen et al. 2001; and Kreyling et al. 2002, both hereby incorporated by reference). Size distribution measurements of ultrafine particles can be made using commercially available scanning electrical mobility spectrometers (SEMS). See Wang and Flagan, 1990, hereby incorporated herein by reference; e.g., TSI scanning mobility particle spectrometer, MSP WPS, and GRIMM DMPS). The scanning electrical mobility spectrometers technique uses a differential mobility analyzer (DMA) (See Kuntson and Whitby, 1975, hereby incorporated herein by reference.) to classify particles based on their electrical mobility and the concentration of the classified particles is, typically measured using a condensation particle counter (CPC). See Agarwal and Sem, 1980, hereby incorporated herein by reference. In the scanning electrical mobility spectrometer instruments, the voltage required for particle classification is exponentially varied to obtain size distributions in a relatively short time (˜5 minutes). Faster size distribution measurements are possible by combining the electrical-mobility classification technique with an electrometer detector array. See Matisen, et al., 1992; Mirme, 1994; Tammet et al., 2002; and Biskos et al., 2005, all hereby incorporated herein by reference. Commercial instruments based on this technique include the FMPS (TSI Inc.) and DMS500 (Cambustion Inc.). The availability of these instruments has made ambient ultrafine particles measurements possible in near real-time and at high size resolution.
Accurate estimation of human health effect of ultrafine particles requires size distribution measurements considering their spatial and temporal variability. See Buzorius, et al., 1999; Shi, et al., 2001; and Zhu, et al., 2002, all hereby incorporated herein by reference. Such measurements require the deployment of instruments over a large number of sites or on a mobile platform, but the large cost, size, and power requirements of the existing instruments make such deployments difficult. Existing portable instruments such as the TSI 3007 CPC and the combination ion-optical sensor (See Litton, et al., 2004, hereby incorporated herein by reference) provide a measure of ultrafine number concentration, but no sizing information. For real-time, size-resolved ultrafine particles measurements applicable for large spatial-scale or personal exposure studies, there is a need for a new family of instruments.